I WISH my idle would hunt and the engine would stay running. 86GT auto I have replaced or swapped just about everything and it still will not idle. When hot it stalls as soon as I lift my foot and has to be brake torqued at every light. I'm starting to hate it after all this time working on it with no improvement. Even when it does idle the slightest load, such as pressing a window switch or turning on the blower will shut it down. This is an engine that gets 22-25 mpg, pulls like a train all the way to 110, starts every time etc. Just will not idle. The only thing I think I haven't changed is the O2 sensor and that was changed by the previous owner. Sets no codes. I have had it hooked up to WinALDL but can't make sense of the readings so that was a waste of time. I'm open to ideas including demolition.

Sounds like you have a voltage/ground problem.
You might read thru my three "using a meter" posts and try some of the readings there.
Just do a search on "Ultimate meter guide" and you will find them. They are all linked. The Second and thrid ones have what you might want to try.

Dodgerunner, a very good write up. I have a dvm and I'll try some of the tests you have outlined. I have long suspected that my IAC is not responding rapidly enough, or is just not getting signal from the ECM. What would be a good way to test the entire IAC circuit? Btw I recently added a 4 gage ground strap from the chassis to the engine with no improvement, also tested resistance of both battery cables, finding nothing wrong there. But obviously I need to do more.

Might try this.

My shade tree test for IAC's has been to pull the iac out, reconnect the plug, hold the iac in one hand with your finger(s) also holding the pintel so it does not shoot out of the iac motor. (don't want to loose the little bugger) Have someone start the car and place a finger of your other hand into the iac hole in the TB and simulate the iac to control your engine idle. You can vary the engine speed with your one hand as you watch the iac drive in and out. (if it is working correctly) If it is working be sure and drive it in most of the way so you can reinstall it.
If you can't do this and make it work you need to trouble shoot what the issue is. You could also log with winaldl while doing this and see what your iac counts are doing.

Dodgerunner, when I had my car hooked up to WinALDL, the IAC showed 38 steps at what was passing for idle then. I have the rest of the data from that check but it was a long time ago and some things have probably changed. I'll try your shade tree idea. Oh, do I have to be under an actual tree when I do it?

Dodgerunner you have a PM.

Does anyone know where I can find a schematic of the 2.8 IAC system that gives wire colors and also expected voltages? Any help would be appreciated.

the ecm links here have the pinouts for the ecm with the wire colors, I think the colors stay the same tho the ecm pins change at one point:
http://home.att.net/~subzero350/

The voltage is a pulse , I think it is 12v , and depends on the windings engergized which way the motor turns. it is a stepper type, and will be hard to meter out.polarity across either of the windings will reverse, depending on which way the motor is stepping, the only real way to see if the voltage is correct is with a scope, a meter will not usually catch it.

The fsm says to short diag with engine off, and you can use a test light to ground from each of the 4 pins. Light should be steady or flashing, if it is off you have a wiring problem back to the ecm.
Resistance of the iac motro is 20 ohms or more measured from pins a to b and c to d.

[This message has been edited by tjm4fun (edited 07-27-2007).]

tjm4fun, thanks for the info, I think the first thing I'll do is test the wiring for the IAC for continuity, as I've always wondered if the darn thing is even moving at all.

Suspicions confirmed. Ran Dodgerunners shade tree diagnostic (used a maple for shade) removed IAC and with engine running varied engine speed with finger. The pintle never changed position even tho I could feel something thunking. The IAC is new and worked just fine in my buddies car so I'm guessing its in the wiring somewhere. Dodgerunner, any suggestions on where to go next? Btw I will be rereading your ultimate meter guide.

any updates on the up down idel problems??? i have an 88 and its done that bs since i got it..iver since replaced...ohh well the entire engine!!! and it still does it!!!! replaced the fuel pump, dist, ecm, all the sensors....still there...its SO crappy!!!..ive been reading all of this thread and hopeing for a cure...and dirty intake valves...LMFAO come on....mine seems to hunt more when its all warmed up, it will go away if i disconect the 02 sensor and if i put it in gear, or turn on the a/c in park, neutral...same as yours???, oh and since the engswap (3400 conversion) it does it more 300rpm both directions more.

Dodgerunner, I reread your ultimate meter guide, but still can't find what voltage I should see at the IAC with the key on. I have the armrest out and will be checking the IAC wires for continuity tomorrow but would also like to know what voltage SHOULD be there.

The continuity check is your best bet. You will not measure a good voltage since the outputs are just pulses and will be there a very short time and switch between each pair of wires.
Like said above you really would need a scope to check the outputs.

You best bet for checking the IAC would be verify the wires are good, swap a known good motor in then try a different ECM.

Here is some info on stepper motors if you want to know more.
http://en.wikipedia.org/wiki/Stepper_motor

This one is even better.
http://www.anaheimautomation.com/intro.htm

[This message has been edited by Dodgerunner (edited 08-05-2007).]

Thank you. Good stuff. I've already tried other ECM's with no change in the problem, and I installed this IAC in my friends 88GT which idled just fine with it, so I'm hoping I have a bad wire. If not I've run out of places to look.

[This message has been edited by David DeVoe (edited 08-05-2007).]

I hope it is a wire/plug issue.

Dodgerunner, bad news for me, all 4 IAC wires seem to be ok. I got a resistance reading of .007-8 with the meter set to 200 ohms. I'm assuming thats 7 ohms? I was sure hoping to get the big infinity on one of them. I'm pissed now because I really don't know where to go from here on this thing. I'm starting to think that maybe me and the Fiero are not a match. Any ideas from anyone gratefully accepted.

hmmmm well that sound like your wires are good... Was that reading from connector to connector?
And you also check that there was no continuity between the wires with the IAC and ECM disconnected? IE not shorts between the wires.
If either pair has a short it would not work. It has to pulse one pair and then the other to make the motor move.
You might be able to meter the pulses on each pair to see if you can see anything. I would guess the pulse would be very short and 12V's or less but might give you some idea if each could was getting a signal.

IAC works on another car.

You say when you did the "maple tree" thing you felt some thunking so it must be trying but it never moved for you when out of the TB?

I'm running out of ideas. Might check that the ECM has good Ground and Power to it.
Maybe if the pulses to the IAC where weak due to the voltage being low?

This might help is you want to try to read the pulses. You should see pulses on each pair, however they can switch from Neg to Pos. depenting on which way it is trying to move so your meter will be jumping back and forth unless it is only trying to move one direction.

[This message has been edited by Dodgerunner (edited 08-06-2007).]

dodgerunner. Yes, that is connector to connector. I pulled the harness back thru the firewall and checked it. No, I don't know wether or not there are shorts between wire pairs. How would I check that? With the engine running I get 14.4 volts at the battery. I've replaced the batt cables and cleaned the mount points, added a 4 ga. ground, I've swapped ECM's with no change. Won't idle in gear, will barely idle when hot even in neutral, and then the slightest load, ie power window switch, coolant fan, etc. will shut it off. I'm certain now that even if the ECM is reading the increased load, and sending pulses to the IAC, the IAC is not responding for some reason. Btw all the ECM pins are clean and straight, the ECM ground on the tunnel looks brand new. I'm pretty much stumped. I'll end up selling it for next to nothing and a day later the guy will come back and say "Oh yeah I tightened that nut on the framus and now it idles like a rock." I just know its something pretty simple.

I forgot that it would die by any load...

I think your chasing the wrong thing. You have a different problem then the IAC.

Start back at the beginning... You have verified fuel pressure just to be sure?

If using anything electrical kills it it sure seems power related.
I'd start working my way thru the connection points and see what's up.

I'd start by put a meter on the power stud under the C500 connector to ground and watch what the voltage does there at idle (or as low rpm as you can do) and when you run a window.
If that is good then you might try it at the cig lighter and see what happens there.
If you can find an easy point try the power feed to the ecm also.

A window should not make enough load on a working system that the motor dies.

I bet there is some point that the voltage is being lost.

What is the voltage on the battery when running near idle? Should be 14 or more all the time running.


Darn typos!

[This message has been edited by Dodgerunner (edited 08-06-2007).]

Dodgerunner, I get 14.4 volts at the battery at idle. Most of the time it will idle in park or neutral but the idle is VERY tender and like I said the slightest load will kill it. If I blip the throttle when it comes back down it will die, the IAC doesn't seem to catch it. I tested voltage at the ECM some time ago and had 12.01 with the key off and 11.84 with it on, window going down 11.66, coolant fan on 11.2. What if anything does that tell you?? I have not checked all the chassis grounds, but have added one.
The coolant temp sensor is new, but the O2 sensor isn't. I'm pretty much running in circles. My dad used to say "When in danger or in doubt, run in circles yell and shout"

[This message has been edited by David DeVoe (edited 08-06-2007).]

bump

What color is the spark plug spark? It should be bright blue. (electric blue lol) If its yellow or white you could have a weak coil. Easy thing to check just don't get zapped while doing the check.

Hudini, I've never actually checked the spark, but the thing runs so good otherwise, that I just don't see how it could be a spark issue. Just like fuel pressure, I've never checked it but I don't see how an engine can run so well, with so much pull with a fuel pressure issue. Next time I work on it tho I will check the spark. Thanks for thinking of me.

[This message has been edited by David DeVoe (edited 08-07-2007).]

I still recommend doing a pressure test. You do know the fuel pressure varies between idle and off idle.
Normal pressure is about 41-47psi and at idle should drop by 3-10 Lbs.
IE you should have one reading at idle and then should increase if you blip the throttle to cause a load on the engine.

Just a good check of the fuel pressure reg. since is does vary pressure depending on how the engine is operating.

David, I know you have checked tons of things I don't even know how, but the other day I ran into a weird electrical issue that took me about 6 hours to solve and it was a bad fuse. In turn have you happened to take a look and see if you have a blown fuse or two that may be causing weird things to happen? My dash wouldn't light up because my fuse labeled tail/brake lights burnt out... weird things...

If I'm totally off base and crazy I apologize but wanted to help.

Yeah, I've checked the fuses. I don't have any weird electrical stuff, other than the phantom wipe at start-up but hell, everybody has that. As far as fuel pressure goes I'll try to come up with a way to check it. Come to think of it this whole problem is probably a weird electrical thing, green stuff on a contact or a ground that looks good but isn't. I have an engineer friend who has offered to help me with it. Maybe we can find it. Thanks to all for the ideas, I really appreciate the help.

Your probably right, since any electrical load seem to affect it. I just alway eliminate the fuel as a start. The more you elimnate for sure the better idea you have what the problem might be.

I agree Dodgerunner, and I'll check it asap. Is there any way the fuel pressure could just die with the engine at idle?? Could something be killing the fuel pump? The engine quits just as if you'd turned off the key. Or sometimes it does that swoopy thing where the tach drops down near zero and then zooms back up to 1100 or so and then dies.

Question about the origional hunting idle problem. Does anyone with a 2.8 from 1985 have the problem as described in the beginning? After finally taking a look at a 85 Bin file I see the AFR map for open loop has the numbers very different than the 86-87 2.8 engine.

umm bump? never found a solution?

I didn't go back and read the original post, but if it is that it hunts when the mat is 50-80 or something that that..yes the 85's do it..

At least my 3.4 in my 85 sure has started doing it since the temps have dropped in the mornings. But mine does it shortly after going into closed loop. In open loop it does not hunt at all.

hmm my 88 has done it from day one.....i shurly thought that CHANGING THE ENGINE!!! would stop it...nope got worse lol, if you disconect the O2 sensor it will idle correct, or if you put it in gear it will correct, or if in N or P it will correct with the A/C on...seems a "load" makes it act right....have no clue what disconecting the O2 sencor does and why it makes a difference.

When you disconnect the O2 the ecm goes back to open loop and that changes everything related to fuel mix. .

Hunting is an affect caused by a loop system reacting to fast. Adding a load slows the reactions and make it settle down.

Why the cars start to do it after they age is the question... a lot of things are worn.

[This message has been edited by Dodgerunner (edited 09-12-2007).]

well the fact that i have an entirly new engine and it still persits to do it is pointing me towards a sencer of some sort out of whack or something else...i know ive changed my ecm too...thought for sure it would fix it..not

ok i have been following this topic for some time.. i have a 86 se 2.8 (centexindy's) old car.. i had the same problem... i fixed it.. i replaced the one way valve in the brake booster.. it wasnt holding the vacumm in the booster.. so there for a couple seconds on first start up it has a leak.. inturn sending the ecm for a wild goose chase.. this may help yall.. and make sure the lower intake has proper gaskets.. and rocker arms are ajusted right..valve closedand a 1/4 of a turn to 1/2 and you will also notice a lil more pep to take off with this ajustment.

Fixing the vac leak at the booster like that will not solve the origional issue, but it is a great solution for others experiencing a close to the same issue problem.

If I start the fiero in the AM, and temperature is between 50 and 80 it hunts. if it's colder it does not, if it's warmer it does not. IF I start it even for only 1/2 a second, shut it all the way off, then restart it does not. The last one is confusing as I want to know what is getting reset that is changing conditions. Dang this slow aldl data stream! you cant see anything small changing.

Brake booster vac leak does not care about temperatures unfortunately.

The only thing I can think of that might change between a start off start sequence is that when you turn the ecm on each time it reads the sensors and sets the base reading of what they are. The only one that really matters is the MAP.
The ecm takes a base reading so it know what it thinks "normal" pressure is.
Now what happens between the first start and the second. Other than you have applied a pressure difference while it's running not much. If the reading the MAP took during the short run change the flex of the sensor in the Map, then maybe. But changing the MAP to a new one should make some difference also but believe some have tried that and it did not...unless they where just putting another used one on.

The only other thing I can think of is the capacitors in the ecm and a couple other places have charged from a dead state but can't believe that would make a big difference either but one never knows. Capacitance and it's value is something that would change with age.

[This message has been edited by Dodgerunner (edited 09-13-2007).]

ecm keep alive wire? it does kinda seem like the ecm isnt keeping the iac settings..

quote Originally posted by hobodude34: ecm keep alive wire? it does kinda seem like the ecm isnt keeping the iac settings..

Maybe in a case or two but there can't be that may cars with that as a cause. Seem almost every car has the problem, or at least a lot of them....